The simultaneous casting of a plurality of metal strands using moulds of different formats is already known from the prior art. A corresponding method has already been described in the patent DE 891 444. Also, from the article “Modernization and extension of a sheet ingot casthouse”, W. Dietz, K. Erke, Light Metal, 1994, pp. 815-819, a method for casting aluminium ingots with different dimensions and the same thickness of approximately 600 mm is known, in which, in the case of different formats of the moulds, the respective mould filling phase for the mould begins at different times, depending on the format of the mould, so that the metal level necessary for the continuous casting in all moulds is reached at the same time. Only then the continuous casting phase is initiated. The mould filling phase for smaller size moulds is initiated up to 120 seconds later.
Furthermore, from the published patent application DE 42 03 337 A1 a method for continuous casting of a plurality of metal strands having different dimensions is known, in which a pressurised gas is introduced into the mould cavity. Details about the casting practices used are not mentioned, i.e. in particular when and how the mould filling phase is initiated for various formats.
As explained in particular in the article mentioned above, the starting practices, i.e. the casting parameters during the mould filling phase, but also the continuous casting practices, are determined depending on the particular format of the mould. The simultaneous casting of metal strands of different formats is made possible in a simple manner in that the formats of the cast metal strands, for example, have an identical thickness. The withdrawal speed in metal strand casting or ingot casting is determined essentially by the cooling behaviour of the ingot or strand and thus by the thickness of the ingot or of the metal strand. The casting of the moulds, i.e. the mould filling phase, usually takes place as quickly as possible in order to maximise the capacity of the foundry. The same aspect therefore applies to the design of sprue stones. The use of sprue stones in general is necessary to achieve a uniform casting process and to cast a rolling ingot, for example, which has relatively homogeneous properties.
The sprue stones of small format moulds have therefore usually been cast with short depth sprue stones. This reduces waste per rolling ingot, for example, and shortens the mould filling phase. Short sprue stones are also technically possible due to less distortion of the rolling ingots in smaller formats. This has to do with the fact that the shrinkage effects are greater in a large-format rolling ingot than in a small-format rolling ingot. Therefore, the mould filling phase has been previously practiced with different filling rates in order to run through them as quickly as possible. Simultaneously, different, in particular flat, sprue stones were used for small-format moulds, while deeper sprue stones were used for large-format moulds.
However, it has been shown that problems increasingly arise during the multiple casting of metal strands with different formats and mould filling phases individually tailored to the mould format. For example, in the case of critical temperature control, metal can freeze in feed systems, for example in the casting tube or the casting nozzle. Surface defects such as cold runs or, conversely, leakage out of rolling ingots can also occur. In the smaller formats, for example, the metal distributor can freeze to the base plate of the solidifying strand due to insufficient temperature control. This can lead to increased production rejects.
Despite these difficulties, preferably different formats of rolling ingots are cast in order to make optimum use of the furnace capacity of the melting furnace. In particular, if only identical formats are cast, this may present the problem that a larger metal sump remains in the furnace, which cannot be cast to a complete ingot/metal strand.
On this basis, the present invention has as its object to provide a method for continuous casting of metal strands, in particular of rolling ingots made of aluminium or an aluminium alloy, which allows the continuous casting of rolling ingots/metal strands of different formats with a reduced reject rate.